DESCRIPTION (Adapted from the investigator's application): The overall goal of this study is to evaluate the impact of weight loss in lactating women on the growth of their infants. Moderately overweight lactating women who have a body mass index greater than or equal to 25, but less than or equal to 30, will be randomly assigned to either: 1) an experimental group that will receive a weight loss program consisting of energy restriction, exercise and behavior modification for 10 weeks, or 2) a control group that will not change their dietary intake or physical activity during the 10-week period. Energy will be decreased by 500 kcal/day of the subjects' estimated needs. The supervised exercise program will consist of walking 4 d/wk for 45 min/d at 70- 80% of maximum heart rate. The study will begin after one month postpartum. Specifically, the following hypotheses will be tested: 1) There will be no significant difference in the growth of the infants between the two groups, 2) the women in the weight loss program will lose 0.45-1.00 kg/wk and this will be significantly more than the loss experienced by women in the control group, 3) women in the weight loss program will lose significantly more body fat than women in the control group, and 4) women in the weight loss program will increase their cardiovascular fitness level significantly more than women in the control group. Pre- and post-measurements of maternal body composition, cardiovascular (CV) fitness, 3-day weighed dietary intake, and infant weight and length will be made. In addition, maternal weight and infant weight and length will be measured weekly and dietary intake biweekly. After completion of the 10-wk intervention, weight and skinfold measurements will be recorded on subjects monthly until one year postpartum. Subjects will also be encouraged to continue their weight control program and asked about their physical activity, behavior changes and dietary intake during these monthly measurements. This information will be used to determine if the weight control program was effective in maintaining the weight loss among women during the first year postpartum. This study will provide important information needed to identify the level of energy intake that is too low to support lactation as well as aid in establishing guidelines for women who want to breast-feed but who also want to restrict their energy intake to lose weight.